‘Yellow peril’ threatens wildflowers

By on April 9, 2012

There once was a wonderful wildflower site near Aledo, Texas, along Iona Road. It was a cattle-range field that I would drive past each spring to watch the glorious diversity of Texas wildflowers color the landscape in rainbows of bluebonnets and orange paintbrush, and a dozen other wildflowers all blooming in natural diversity.

Rapistrum rugosum in spring. Photo by Marilyn Sallee

One year I noticed stands of bright yellow pop up here and there – tall airy globes of yellow sparsely scattered in the acreage. The year after that the field was solid yellow; if there were other wildflowers, they were hidden under the onslaught of this one aggressive plant. While the kaleidoscope of Texas wildflowers normally run just a foot or two tall, in two short springs this field was taken over by Giant Mustard growing three to five feet tall and shading out all the native flowers.

It happened that fast, and it is happening all over Texas. Giant Mustard, Bastard Cabbage, Wild Turnip-weed — they are all the alter egos of the bully of the mustard family, Rapistrum rugosum, also known as RARU to the invasive plant hunters from its official USDA designated symbol.

This year has been especially splendid for the wildflowers, with huge stands of bluebonnets lining the roadsides. But in so many locales, the fields of deep blue are edged with tall borders of the airy yellow waist-high hedges of RARU. As the bluebonnets flourish, so do the invasives.

Rosette of Rapistrum rugosum. Photo by Marilyn Sallee

However, the native bluebonnets disappear after their spring show. The RARU drops its seeds and develops a basal mat of dense leaves that out-competes native species and quickly forms a monoculture. With the ground well-mulched by the RARU rosettes, other seeds in the soil have no chance of germinating next year. The RARU wins.

Especially in disturbed soils, along roadways and median strips, the dense mat of basal leaves give way to tall airy stems filled with clusters of yellow four-petal flowers that totally take over. The plants are tall and dense enough to block sightlines across the median of a divided highway and makes pulling off the shoulder of the road dangerous.

Any flower is a good flower?

One of the more unusual RARU sightings I’ve had was at a tree farm. Here the trees rule and workers keep the ground well mowed. But at the end of one row, a short fence protected a three-foot high RARU in full bloom so the workers wouldn’t mow it. I asked why and was told that any pretty flower was welcome and protected. The following spring about a quarter-acre was covered with RARU and their deep taproots taking nutrients not just from the grasses, but even the trees. This year the area all along the tree farm is edged in RARU.

Portrait of the aggressor

Not just common, but prolific, RARU originated in the Mediterranean area and belongs to the mustard family. The old family name of Cruciferae, or “cross-bearing”, came from the four-petal yellow flowers, forming a cross-like shape. Broccoli, cabbage and mustard all have these yellow cross-like flower clusters indicative of the mustard family. RARU is a tough plant with a deep taproot to survive drought. It blooms through spring and summer, and then forms stalked seed capsules containing the tiny brown seeds. The seeds germinate in late fall or early winter and make leafy rosettes that form dense mats to block the sun from reaching any other seeds in the soil. RARU has the median strip or road shoulder to itself.

What you can do

The easiest way to control RARU is simply to keep it mowed to prevent it making seeds. It is an annual plant which only lives one year; stopping the seeds from forming stops next year’s crop. If the stand of RARU has not spread too far, you can also remove the entire plant, including the tap root, to control it. In the late fall or early winter, the rosettes can be sprayed with herbicides. (Always use herbicides only according to directions.) Once the RARU is removed or killed in fall, over-seed with natives. Gaillardia/Indian Blanket is an especially a good choice for over-seeding against RARU.

If left uncontrolled, RARU spreads fast and far. Wherever the ground is disturbed, by new construction or wildfires, drought-stricken fields or parched lawns, road construction or laying new pipes, RARU seeds will take hold and block native grasses or flowers from germinating to fill the void. The same conditions that made the wildflowers such an awesome display this year also favored the RARU. Stop it from flowering this summer and watch for and remove the rosettes in the fall. The fewer RARU invasives we have, the more wildflowers can flourish.